Rebtel's stable of mobile VoIP apps expanded by one this week as the Swedish service provider released a Windows Phone app. Available in the Windows Marketplace, the free app requires a Windows Phone 7.5 or later (Windows Phone 8 is the current model).

The Windows Phone app follows closely on the heels of the iPad app for Rebtel launched at the end of May. The release order for Rebtel's mobile VoIP apps is not all that surprising: Although the Windows Phone is predicted to represent nearly 21% of the smartphone market by 2015, according to analysts at International Data Corporation, researchers at Strategy Analytics have forecasted that the phone will gain only 1% growth this year to reach 5% overall by 2012.

The VoIP service provider, second only to Skype in popularity and currently boasting over 17 million users, offers inexpensive calls to any phone (landline or cellular) at rates as much as 60% lower than Microsoft-owned Skype. Rebtel's primary strategy for inexpensive VoIP calls is a local number network. The network is available in more than 50 countries.